Clean the garbage can really good, removing any oily residue, then wipe it down with white vinegar. The vinegar will etch the surface. If the can is new then the prep is done. If there is any rust then this need to be dealt with using a rust neutralizer.
Now you will need to prime and paint. I would use spray cans as this will be easier. A good universal oil based exterior primer is needed and a good quality paint for the finish coats. Applying 2-3 thin coats of each is better than 1-2 thick coats.